The present invention relates to a torque distribution control system for a four-wheel drive motor vehicle having a central differential, and more particularly to such a control system in which output torque of a transmission is unequally distributed to front wheels and rear wheels in accordance with driving conditions of the motor vehicle. A differential operation restricting clutch is provided in the central differential for restricting the differential operation. The torque distributed to the front wheels and rear wheels is controlled by controlling the restricting torque of the clutch.
It is known that driving performance of a motor vehicle differs with the type of power transmission system. For example, in a four-wheel drive motor vehicle, the four wheels are driven to prevent slipping and skidding of the wheels, thereby improving driving performance in traction, braking, and steering. The acceleration or deceleration influences the front wheels and the rear wheels at the same time, so that both understeer and oversteer of the vehicle are reduced.
Furthermore, in the four-wheel drive motor vehicle, the torque distribution to the front wheels and rear wheels and to the left rear-wheel and right rear-wheel affects the steerability and the change of running behavior. If the torque distribution is properly controlled, driveability and dynamic stability are further improved. Consequently, it has been proposed to properly and variably control the torque distribution in dependency on the driving conditions.
Japanese Patent Application Laid-open 63-13824 discloses a torque distribution control system having a central differential for a four-wheel drive motor vehicle. In the system, a fluid operated multiple-disk friction clutch is provided in the central differential for restricting the differential operation. The torque distribution to the front and rear wheels is controlled by controlling the restricting torque. The cornering behavior of the vehicle can be detected by lateral acceleration. If the lateral acceleration increases, the gripping force of the tire gradually reduces to a breakaway point, at which the tires begin to slide sideways to cause spinning or drifting of the vehicle. Therefore, the restricting torque is set in dependency on the lateral acceleration for controlling the torque distribution to the front and rear wheels, thereby preventing the sliding of the tires.
However, the cornering condition can be detected only in a linear zone where the side force changes linearly. When the gripping force of the tire approaches that of the breakaway point, the side force changes irregularly. Consequently, the actual lateral acceleration changes irregularly with the behavior of the vehicle. Therefore, the system can not prevent the vehicle from spinning and drifting in the non-linear side force zone.